This invention lies in the field of seismic processing. More particularly, it concerns the determination of weathering or static corrections between records recorded from sources at adjacent source points, or between traces to adjacent geophone points.
While this invention can be used with any type of source it is of greatest value in relation to sources which are initiated at the surface of the earth, such as the Vibroseis, the Dynoseis, the thumper, and others. It will for convenience be described in terms of the vibratory source systems.
In the seismic process of geophysical prospecting, wherein a vibratory source generates an alternating pressure on the surface of the earth at a first source point, it is important to know the time of travel of the seismic wave from the surface through the weathered layer.
More correctly, it is important to know the difference in the times of travel through the weathered layer of the downgoing seismic waves generated by the source at each of two adjacent source points. This is needed to properly interpret travel time differences to reflecting interfaces, so as to determine the slope or the dip of those reflecting interfaces. These differences in time throuh the weathered (or weathering or low velocity layer (LVL) are called the fixed or static corrections.
A. In the case of dynamite shots, the travel time of the seismic wave through the weathering, or the depth to the base of weathering, can often be determined from measurement of the seismic pulse from the deep shot to a surface geophone at the shot point. This is called the uphole time. Of course, this method cannot be used with surface sources.
B. Another qualitative method involves determining static corrections by adjusting travel times so that a strong shallow reflection, which is believed to be flat, will be shown on the record as a true horizontal reflection.
C. Another qualitative method that has been used involves adjusting the travel times of a plurality of travel paths from different source and receiver points to the same common reflection point.
Vibratory seismic systems and other systems that use seismic sources at the surface of the earth, are limited in their capability to determine static corrections, because there are no drilled holes, and no subsurface sources.
The method of determination as described in B and C above are not reliable, since basing the corrections on deep reflection information involves many variables, few of which are known. It is clear, therefore, that there is no reliable method in use today for determining the static corrections.